In U.S. Pat. No. 3,391,543, an apparatus and technique were disclosed for installing such rods in earth formations, including ones which were unstable. The rod was inserted in the longitudinally extending bore of an elongated tubular casing installed in a tunnel in the earth formation, and then the casing was retracted from the tunnel while a piston device was relatively advanced telescopically through the bore of the casing behind the rod, to "extrude" the rod into the tunnel relatively tandemly ahead of the casing. Initially, the casing was capped by a tunneling tool used to install it in the earth formation, but once the rod had been inserted in the bore of the casing, the tool was removed and the rod was relatively advanced into the tunnel from the casing at the distal end of the bore as indicated. Moreover, where the earth formation was unstable, a pressurized liquid medium was charged into the bore about the rod to stabilize the formation while the rod was dismounted in the tunnel. The pressurized liquid medium was admitted to the bore through a check valve in the piston device, and in addition, was replenished from time-to-time to maintain the charge in the tunnel while the operation continued. Ultimately, when the rod had been dismounted in the tunnel, relatively tandemly ahead of the casing, the charge was discontinued and the casing was removed from the mouth of the tunnel to leave the rod encased in the earth formation, except perhaps for the proximal end of the rod which was exposed for use at the face of the excavation.
While the patented apparatus and technique were highly advantageous in their own right, they did have the disadvantage that the bore of the casing had to be uniform in cross section throughout its length, so that the casing and the piston device could be reciprocated in relation to one another for the full length of the casing.